Metal oxides are compounds in which oxygen is bonded to metal, having a general formula MmOx. They are found in nature but can be artificially synthesized. In synthetic metal oxides, the method of synthesis can have broad effects on the nature of the surface, including its acid/base characteristics. A change in the character of the surface can alter the properties of the oxide, affecting such things as its catalytic activity and electron mobility. The mechanisms by which the surface controls reactivity, however, are not always well characterized or understood. In photocatalysis, for example, the surface hydroxyl groups are thought to promote electron transfer from the conduction band to chemisorbed oxygen molecules.
Despite the importance of surface characteristics, the metal oxide literature, both scientific papers and patents, is largely devoted to creating new, nanoscale, crystalline forms of metal oxides for improved energy storage and power applications. Metal oxide surface characteristics are ignored and, outside of the chemical catalysis literature, very little innovation is directed toward controlling or altering the surfaces of known metal oxides to achieve performance goals.
The chemical catalysis literature is largely devoted to the creation of “superacids”—acidity greater than that of pure sulfuric acid (18.4 M H2SO4)—often used for large-scale reactions such as hydrocarbon cracking. Superacidity cannot be measured on the traditional pH scale, and is instead quantified by Hammet numbers. Hammet numbers (H0) can be thought of as extending the pH scale into negative numbers below zero. Pure sulfuric acid has an H0 of −12.
There are, however, many reaction systems and many applications for which superacidity is too strong. Superacidity may, for example, degrade system components or catalyze unwanted side reactions. However, acidity may still be useful in these same applications to provide enhanced reactivity and rate characteristics or improved electron mobility. However, the battery literature teaches that acidic groups are detrimental in batteries.